As Internet usage continues its growth, carriers continue to see a steady increase in packet data traffic on their metro networks. Cable providers, or multiservice operators, are experiencing similar trends, with further expansion into business-customer segments only amplifying the trend.
Since packet data is more efficiently transported using packet switching rather than circuit switching, the sheer economics of efficient data transport are forcing carriers to reconsider their current network model, which caters extensively to circuit-switched voice. To avoid the expense of laying new rings for packet data, carriers now have the option of using packet-switched Ethernet services over their metropolitan-area networks based on circuit-switched Sonet (synchronous optical network). Two ways to accomplish that are multiplexed or switched Ethernet-over-Sonet, with the switched version now emerging with clear advantages in terms of scalability and cost.
Carrier's packet options
For many years now, carriers have architected their networks around voice and circuit services in order to satisfy legacy demand from their customers. As carriers consider offering data services, two scenarios have emerged: They can use their existing networks in a more efficient manner, or they can build new infrastructure to add packet data to their service portfolio.
When no new rings are being deployed, carriers have expressed a need for efficient solutions that can add packet data traffic to existing Sonet/SDH (synchronous digital hierarchy) networks. Such solutions must overcome network inefficiencies that surface when data traffic, inherently bursty and connectionless in nature, is directly carried over the fixed, nailed-up circuits traditionally used for transport of legacy voice and circuit services. Nonetheless, finding a means to carry packet data efficiently on Sonet/SDH networks enables carriers to save on valuable capital and the operational expenses of building a new network.
There are two ways to efficiently deploy packet data services on existing Sonet/SDH networks: with switched or multiplexed Ethernet. However, if fiber is plentiful and available, carriers also have the option of adding an overlay network solely for transporting packet data. Their existing network is used as a separate Sonet/SDH transport for legacy voice and circuit services.
Standard Ethernet can be deployed across metropolitan networks over dedicated, point-to-point fiber connections. Point-to-point Ethernet, delivered over a dark fiber, is a low-cost entry point for delivering packet data services. A second technology option is to implement a resilient packet ring (RPR), a new protocol optimized for packet data that is making its way through the IEEE standards bodies. Both approaches to transporting packet data force carriers to deploy a separately managed overlay network. While building new infrastructure to add packet service might look promising, in light of growing Internet usage, profitability from such networks is still years away.
Resilient packet rings
A technology similar to Sonet/SDH, RPR optimizes sharing of fiber-optic rings for packet data traffic. RPR has set out to solve problems from which older Sonet/SDH solutions suffer in supporting packet data. For example, RPR uses both primary and backup rings as compared with Sonet's typical use of one ring as a backup. In addition, by stripping packets at destination nodes, available downstream bandwidth may be reused. Multidrop limitations of the point-to-point nature of Ethernet are also overcome by deploying ring architectures.
Running RPR over dark fiber requires building a new network, since new fiber needs to be lit. While fault management and performance monitoring are well-understood in the Sonet/SDH world, RPR is still making its way through the standards bodies. Also, while RPR claims to support time-division multiplexing (TDM), that claim is unproven, since there are no live networks implementing RPR that carry legacy voice and circuit traffic.
When it comes to transport protocols, RPR is still the new kid on the block. Carriers have to test its performance and validity to see how it handles packet data traffic as well as legacy voice and circuit services.
While RPR implementations are currently proprietary, Agere, PMC-Sierra and Agilent have already announced commercially available chips for standards-based mapping of Ethernet-over-Sonet. Next-generation Sonet equipment, using commercially available chips, could soon be available for Ethernet services in metro networks.
EoS implementations
Running Gigabit Ethernet over dark fiber in the metro requires building a new network. New fiber must be lit to carry native Ethernet on the metropolitan-area network (MAN). While Ethernet is now a standard protocol in local-area networks across the world, there are significant shortcomings when it comes to deploying Ethernet natively across the MAN. First, while Ethernet ports are inexpensive compared with other technologies, Ethernet in the MAN is a point-to-point technology, and new fiber needs to be lit whenever a new node is added to the network. That can prove to be very expensive.
Second, restoration time for native Ethernet implementations is on the order of 1 to 5 seconds. That compares poorly with the 50-millisecond restoration times that Sonet offers. While delivering on the promise of low cost, this solution has limited value in a true carrier-class environment that demands a resilient network with full protection and 99.999 percent availability.
Sonet/SDH, however, provides these capabilities while offering a highly resilient, fully managed infrastructure with robust back-office provisioning, billing and management systems already in place. For most carriers, therefore, the challenge is to leverage their existing resources and protect their current revenues while stepping up to the cost, flexibility and service-level demands of new packet applications.
Whether a carrier uses existing infrastructure or has plans for new buildouts, Ethernet packet data traffic may be multiplexed over Sonet/SDH rings. Both packet data and legacy voice and circuit services may coexist on metro networks implementing multiplexed Ethernet-over-Sonet (EoS).
In a multiplexed EoS network implementation, a Gigabit Ethernet port on a source node behaves as an extension of the Sonet/SDH circuit that connects the source to a destination node on the ring. Figure 1 shows a four-node metro network. All traffic on that Ethernet port is multiplexed onto that one circuit. For each additional circuit on the ring that connects a source to a new destination node, an Ethernet port must be reserved on the source node. As the number of destination nodes in the network grows, the need for additional ports causes an exponential increase in the cost and complexity of the network. Each source node requires installation of three Ethernet ports in order to switch traffic to each of the three destination nodes.
An alternative to multiplexed EoS is switched EoS, which enables each Gigabit Ethernet port to switch traffic to multiple destinations on a metro ring at wire speeds. Carriers are no longer required to hardwire Ethernet access ports to Sonet/SDH circuits on the ring, and the technology may be used over both existing metro networks and new fiber. Also, switched EoS makes it possible to scale networks linearly to the amount of packet data traffic added to them, as opposed to incurring up-front port costs that increase exponentially with the number of destination nodes in the network. Figure 2 shows the change in Gigabit Ethernet port count on a single node as the number of nodes in a network increases.
Using switched EoS, a node needs just one Ethernet port no matter how many other destination nodes are on the network. In a multiplexed EoS architecture, that port-count explosion has to be replicated in all the nodes in the network, adding to overall cost.
Switched EoS requires a standby backup ring for resiliency against network faults. In addition, unlike RPR, the use of Sonet paths for switched EoS prevents metro rings from using any available bandwidth between nodes. However, ring re-use can easily be accomplished using EoS by implementing RPR to run over Sonet rings. This allows carriers not only to enhance existing networks to add packet-data services but also enables them to reuse available bandwidth in the ring.
Bandwidth management
A type of Sonet/SDH circuit called a virtual tributary of 1.5-Mbit/second granularity (VT1.5) can be used to achieve bandwidth granularity and flexibility on Sonet/SDH networks. Virtual tributaries may be created not only to minimize wastage of ring bandwidth but also as a static means of controlling the volume of traffic a user adds to the network. However, such circuits increase the network overhead.
Using switched EoS, multiple Gigabit Ethernet-TCP/IP flows can be mapped in 64-kbit/s increments into a Sonet/SDH payload, thereby significantly increasing ring efficiency without requiring multiple low-bandwidth circuits to control traffic volume. Each TCP/IP flow may be individually rate-limited, thereby allowing carriers to control the network usage. Moreover, switching TCP/IP flows without nailed-up circuits allows statistical multiplexing of traffic on Sonet/SDH networks. This ability to multiplex traffic statistically onto existing Sonet paths vastly improves the economics of offering Ethernet services over the MAN. In addition, switched EoS enables TCP/IP traffic prioritization to control network usage based on port, virtual LAN, Internet Protocol address or application information.
Commercially available chips map Ethernet into Sonet circuits using standards-based point-to-point Protocol over high-level data-link control for packet-over-Sonet, or standards-based X.86 encapsulation for EoS. These minimize the risks in deploying switched EoS.
Switched EoS enables LAN traffic from many customers to be statistically multiplexed on any Sonet/SDH circuit in granularities as low as 64 kbits/s. Switched EoS also enables Ethernet services to coexist with legacy voice and circuit services in metro networks. For example, traffic from DS-1 or DS-3 or OC-3 to OC-48 networks may be transported on the same metro network.
Adding Sonet/SDH features that enable 99.999 percent reliability, private-line-quality latency and jitter limits, sub-50-ms service failover capabilities, and full media protection to a service-delivery solution gives carriers capabilities that fundamentally change the model for delivering high-bandwidth, high-margin services. With these solutions, Internet access traffic from many customers can be aggregated to a shared service path in 64-kbit/s increments, thereby significantly increasing overall network efficiency.
Rajive Dhar is director of Atoga Systems. He has a BSEE from the Regional Engineering College in Kashmir, India; an MSEE from Syracuse University; and an MBA from the University of Chicago. Rajive can be reached at rdhar@atoga.com.
P.G. Menon is co-founder of Atoga and has a BS in electronics and telecommunications from BHU-IT, Varanasi, India, and an MSEE in computer engineering and operation research from Rensselaer Polytechnic Institute (Troy, N.Y.). P.G. can be reached at pgmenon@atoga.com.
